1. Field of the Invention
The present invention relates to a frame-sequential or a synchronous endoscope and, more specifically, relates to an observation optical system of an endoscope having a blood-vessel-enhancement observation function or a mucous-membrane slice-image observation function.
The present application is based on Japanese Patent Application No. 2004-336195, the content of which is incorporated by reference.
2. Description of Related Art
For frame-sequential or synchronous endoscopes having a blood-vessel-enhancement observation function or a mucous-membrane slice-image observation function, endoscopes capable of carrying out, for example, standard visible-light observation, predetermined-waveband-enhancement observation, blood-vessel-enhancement observation, or mucous-membrane deep-slice image observation are known.
Here, standard visible-light observation uses standard observation conditions for image acquisition using light in the entire visible range. Predetermined-waveband-enhancement observation uses conditions in which a predetermined wavelength band is enhanced by an optical filter or a predetermined waveband is displayed with enhancement by carrying out inter-band calculations on a plurality of acquired color band images. Blood-vessel-enhancement observation is a type of predetermined-waveband-enhancement observation in which a waveband that is optimal for the absorption characteristics of blood is emphasized. Mucous-membrane deep-slice image observation uses conditions in which one mucous-membrane slice image is selected from a plurality of mucous-membrane slice images having different depths and observed.
In the related art, an endoscope showing improved ability to observe a mucous membrane of a living organism by predetermined-waveband-enhancement observation include the technologies disclosed in Japanese Examined Patent Application Publication No. HEI-3-24848 and Japanese Unexamined Patent Application Publication Nos. 2000-262459 and 2001-170009. According to the related art, blood-vessel-enhancement observation, in which the spectrum of illuminating rays is optimized for hemoglobin, is capable of enhanced display of blood vessel flow. However, in the related art, the illumination and the image calculation method after image acquisition are the main issues, and observation optical systems configured to form an image of an object on an image-acquisition device have not been taken into sufficient consideration.
Japanese Unexamined Patent Application Publication No. 2003-215469 describes features related to the spectral transmittance in the vicinity of the pupil of a videoscope objective optical system. However, the aberrations of the objective optical system and the wavelength dependency of the focus setting have not been taken into sufficient consideration.
In a known observation optical system in which only standard visible-light observation has been considered, the observing ability could be improved by providing a design with the smallest aberration possible. However, there was a problem in that, for an endoscope carrying out predetermined-waveband-enhancement observation, such as that discussed in the above-cited related art documents, reducing the aberration may not be the best approach.
In particular, in relation to magnified observation, whose need has been increasing recently, two cases in which the application of a known observation optical system in pursuing reduction in aberration is not desirable will be described below.
(1) In blood-vessel-enhancement observation, since a lesion is diagnosed based mainly on the condition of the capillary blood vessel flow at the surface of a mucous membrane, a high observation magnification is required for resolving a minute pattern. Therefore, in blood-vessel-enhancement observation, it is desirable to use a magnification higher than that used in standard visible-light observation. However, for an observation optical system whose aberrations have been completely corrected, the optical specifications for the blood-vessel-enhancement observation and the standard visible-light observation become the same. Thus, there is a problem in that the magnification cannot be increased.(2) Research on carrying out in-vivo image analysis of a mucous membrane has been carried out. In such an application, an ultra-high-magnification optical system having a significantly narrow depth of field is used, wherein, to prevent blurriness of the image, methods such as bringing the tip of the endoscope into contact with the mucous membrane of the living organism and fixing the position by attaching a tip attachment are used. When the aberrations are completely corrected in such an ultra-high-magnification observation optical system, the focal position of the object does not depend on wavelength. For this reason, it is impossible to carry out the above-mentioned mucous-membrane deep-slice image observation function since image information only on the surface or a single predetermined slice image at a certain depth can be obtained. Under these conditions, image analysis of each mucous layer and/or invasion diagnosis of the lesion cannot be carried out. Furthermore, if a focusing mechanism is provided, theoretically, the depth can be changed. However, it is difficult to provide a focusing mechanism in a small-diameter endoscope. Moreover, the focusing operation is difficult for the user to carry out, and, thus, it has little practicality.